1. The Field of the Invention
This invention is directed to coatings to accelerate burning at interfaces between rapidly burning propellants and thermally conductive or endothermic inert surfaces. More particularly, the invention is directed to passive coatings on the interior surface of firearm cartridges, firearm chambers, and solid rocket motors which utilize reflected infrared energy to accelerate the sidewall burn front.
2. The Background Art
Firearm technology has advanced from the early muzzleloader wherein black powder and projectiles where separately loaded into the muzzle of a firearm barrel. Modern firearms use a cartridge which includes a case, housing a propellant, a primer, and a projectile. Cartridges have greatly reduced the frequency of misfires that were commonly experienced with case-less ammunition. For rifle and handgun ammunition the case is typically metallic, such as brass. A case may or may not utilize a shoulder disposed below a case neck. The case neck retains a projectile. Configured with a shoulder, the case body may have a larger interior diameter than the projectile. For shotgun ammunition, the case is typically paper or plastic with a metal head and is called a shell. The primer is the ignition component which is affixed to the case in a manner to be in communication with the propellant through a flash hole. The primer includes pyrotechnic material such as metallic fulminate or lead styphnate and may be located within the center base of the case or on a rim.
The rear portion of a firearm barrel includes a chamber which is designed to receive the cartridge. The firearm includes a firing mechanism that drives a firing pin or an electrical charge to ignite the pyrotechnic material in the primer. A combustion process is initiated within the cartridge when the primer ignites. Hot high-pressure gases and particulates are produced by ignition of the primer pyrotechnic. The gases exit through a flash hole or holes into the case, which contains the propellant and trapped air. The propellant is typically a combustible powder having various configurations of granules or grains. The propellant and entrained air not ignited by the primer-blast is compressed into a solid mass having the characteristics of a very viscous fluid.
Firearm cartridges are divided into two basic types, straight-walled and bottlenecked, which are distinct in shape and function. Straight-walled cases are so named because they have a cylindrical or slightly tapered shape with an inside diameter equal to or slightly greater than the projectile diameter. Bottlenecked or shouldered cases are so named because they taper from a base to a conical shoulder and neck which holds the projectile.
The straight-walled and bottlenecked two cartridge shapes have distinctly different combustion characteristics and efficiencies. In the straight-walled case, propellant that was not initially ignited by the primer, burns from the aft, or flash hole, end forward with most of the propellant following the projectile into the barrel bore. The propellant along the case wall, although sheared away from the case wall by projectile movement, may not ignite because the case wall has up to 500 times the thermal conductivity of the propellant and significantly greater specific heat. This has the effect of cooling and quenching ignition at the case wall in addition to causing significant heat loss to the gun chamber.
Acceleration losses are high as the entire propellant body accelerates down the barrel behind the bullet. Powder burn rates must be very fast to minimize such losses. Any propellant not consumed before the projectile leaves the muzzle will be expelled and cannot contribute to projectile acceleration. Heat loss caused by burning propellant in the barrel is very high.
The bottlenecked or shouldered case is somewhat more efficient. As propellant is ignited at the primer flash hole or holes, a shock wave moves through the propellant that compresses and heats the propellant. The shock wave is partially reflected off the case shoulder toward a central interior portion of the case. As pressure behind the shock wave begins to move the projectile, a propellant plug approximately the diameter of the projectile is sheared away from the body of the charge. Ignition along the resulting shear surface is rapid because only an infinitesimal gas path out of the shear layer exists causing a rapid pressure and temperature buildup. The portion of the propellant plug which is exposed to the case neck can only burn from the aft end forward due to the quenching effect of the case neck and later the barrel bore.
Burning rates for propellants used in the bottleneck case must be slower because of the additional burning surface of the propellant plug and exposed propellant sheer surface. In the region where unignited powder exists, exposure of the case wall to combustion gas occurs when the propellant is consumed. As this material burns forward from the base and through from the interior surface, more of the case is exposed to direct heating, therefore, heat loss increases. Thus, heat and acceleration losses are lower with the bottleneck case but are still excessive. Ballistic calculations utilize empirically derived coefficients known as progressivity, regressivity, and vivasity to define the pressure in a cartridge as a function of time or bullet movement. However, the burning rates and surface areas of the propellant are not quantitatively defined.
In firearm manufacturing, it is desirable to increase the propulsion of the projectile for improved range and accuracy. Projectile velocity and propulsive efficiency have been increased through the use of high energy smokeless powders. Other improvements have resulted from increased case capacity, improved primer design, and better metallurgy for cases and firearms with higher operating pressures. The shape of the case has also been altered, as discussed above, to create the bottlenecked case that increases case capacity to reduce heat and acceleration losses. Improvements thus far have relied upon empirically derived coefficients that do not accurately model pressure over time. Thus, such improvements fail to provide an optimal configuration.
In improving a cartridge several design parameters must be considered within the framework of the combustion process described above. One parameter is to minimize heat losses to the cartridge case, projectile base, and gun barrel. This may be done by protecting cartridge surfaces from combustion heat where possible. Heat losses may also be minimized by reducing the interior surface area of the case as much as possible for the required propellant volume. Another parameter is to maximize the pressure-time integral of propellant combustion within pressure limitations of the firearm design. A further parameter is to complete as much combustion as possible within the cartridge case to minimize heat loss and damage to the firearm barrel. Yet another parameter is to minimize acceleration of uncombusted propellant to conserve combustion energy.
Historically, a great amount of work has been expended to retard or prevent the burning of substances such as solid rocket and gun propellants at the interfaces of the propellant and its container. This has several advantages such as reducing heat loss, preventing damage to the pressure enclosure and controlling the amount of burning surface as a function of time. If it became necessary to advance the burn front in a particular area an active accelerant, such as nitrocellulose lacquer coating described in GB patent number 014678 to Newton, was used to ignite the propellant exposed to it. This has the disadvantage that thermal insulation may be required to protect the underlying surface.
It would be advancement to the state of the art if an inert coating could be utilized which would reflect combustion energy into the interface between the propellant and container thereby advancing the local burn front, while still providing insulation to the underlying container.
It would be a further advancement in the art to improve the propulsive efficiency of a cartridge. It would be an advancement in the art to increase bullet velocity for a given amount of propulsive medium, such as gun powder. It would be a further advancement in the art to minimize heat and acceleration losses within the pressure limits of the firearm and minimize damage to the bore of the firearm barrel. It would also be an advancement in the art to be able to calculate pressure as a function of time directly from propellant burn rates and surface areas without resorting to empirically derived coefficients.
Such passive reflective coatings and cartridge and case-less gun chamber designs are disclosed herein.
This disclosure describes passive coatings for accelerating sidewall burn fronts in gun cartridges, gun chambers, and solid rockets. A series of coatings is described herein which, when exposed to infrared energy, reflect a portion of that energy back into the interface of the coating and propellant, heating the propellant to increase the local burn rate and thereby advance the burn front in that area.
This technology can be applied to either gunpowder or solid rocket propellants. Guns typically utilize a brass cartridge case or steel chamber to contain the propellant and combustion gases. The thermal conductivity of the brass, or similar metal, case is more than 500 times as high as the gunpowder and the steel chamber is more than 200 times as high. Thus, ignition of the propellant is effectively prevented or quenched by the high thermal conductivity of the metal in contact with it. Coatings, which have a thermal breakdown temperature below the ignition temperature of the propellant, will also retard ignition of the propellant in contact with it because of the large amount of thermal energy absorbed by the endothermic breakdown of the coating.
While the total amount of heat transfer is small because of the short time periods involved, the local effect on the propellant surface and differential ignition rates at the interface are large.
A high temperature resistant, reflective coating can be utilized on the inside of the brass case, or steel chamber (for caseless ammunition or solid rockets) and the chamber side of the projectile. This coating having high reflectivity in the infrared range and a thermal breakdown temperature higher than the ignition temperature of the propellant, will accelerate the ignition of the propellant in contact with it rather than quench or reduce it. The total burning surface will be increased and the internal ballistics of the gun (or solid rocket) altered accordingly. Heat loss to the metal structure will also be reduced, and the cartridge case or chamber may be fired or reused several times until the coating is degraded.
In addition, the mode of propellant combustion and a design process for the design of metal cased cartridges and for case-less gun chambers for all gun sizes are described. In one embodiment the firearm cartridge has a case configured with a straight-walled portion that is connected to a base. The straight-walled portion defines a base cavity having an interior base diameter and containing a propellant. The case further includes a radial shoulder connected to the straight-walled portion. The radial shoulder transitions into a non-radial neck/shoulder junction that connects the shoulder to a neck. The interior base diameter is at least twice the neck diameter. A bullet is partially nested within the neck.
A case-less gun chamber may be configured similarly to the cartridge. As such, the chamber would have a base diameter that would be approximately two or more times the size of a neck chamber. The chamber would include a radial shoulder that would be connected to the neck through a non-radial neck shoulder junction.
The two to one or greater ratio of the base diameter to neck diameter optimizes combustion efficiency. The increased diameter creates a greater primary ignition zone and reduces heat loss by having a thicker layer of propellant on the interior case surface until burnout. Acceleration losses are reduced as the length of the propellant plug is reduced. The case dimensions further provide for simultaneous burn in the propellant plug and propellant wall to reduce inefficiency and waste. This results in more propellant burning in the neck and case interior rather than within the barrel. The radial shoulder focuses a shockwave just far enough from the bullet base to reduce heat loss to the bullet and support bullet retention in the neck for a longer period of time.
The neck, case wall, and the bullet base may further be coated with a reflective, insulation coating to reduce quenching of the propellant adjacent the neck and bullet base. The coating accelerates burning fronts, reduces heating and acceleration losses, and further adds to the propulsive forces behind the bullet base.
In another embodiment, the invention includes a straight walled cartridge with the reflective, insulation coating disposed on the case interior. The reflective coating may further be disposed on the bullet base. As mentioned above, the coating reduces quenching of the propellant adjacent the case and the bullet base. This increases the propellant burn front along the shear surface at the case wall and the bullet base as the bullet moves forward.